Childhood and adolescence are pivotal periods for mental health. The hypothalamic-pituitary-adrenal axis is the primary stress system and its chronic activation is measurable via hair cortisol concentration (HCC), indicating long-term stress exposure. While HCC is linked to adult mental health, this relationship remains unclear among youth. Although a bidirectional relationship is possible, the impact of mental health problems on HCC in youths has been comparatively underexplored. This study…